Welcome to the digital design lab! Today's task is to build a half-adder -- a simple adding circuit that will be presented during the lab time. You should begin small by constructing the pieces needed, and build your way to the entire half-adder.
Don't do anything yet! We're going to spend a few minutes going over the various chips, boards, power supplies, and other devices that you'll need to use for this and other labs. Here's a list of the devices you'll be using, and some key information about them:
Breadboard and the ETS-7000: These big units have a number of parts, some of which we'll never use. Their key feature is the breadboard---white boards with lots of little holes in them. These will allow us to structure our circuits, as we'll have to plug all of the chips and wires into them to make them work. Each breadboard has power and ground channels that provide the digital zeros (0 volts) and ones (+5 volts) that we will use as inputs into gates. More on these below. The breadboard also has wells that chips straddle, with independent rows of connected holes that allow you to connect a wire to a chip pin by putting that wire in the same row.
Note that the ETS-7000 also contain a number of devices around the periphery for input and output. For now, we are interested in the eight switches at the bottom that allow us to easily select power and ground inputs; we are also going to use the eight LEDs in the upper-right corner, allowing us to see the value flowing out of our circuits.
Power channels: There are several power channels on each board, marked by red or blue/black lines. The colors respectively mean power and ground. There should be wires from the posts to two of the power channels. Each power channel is electrically connected inside the board, meaning that once a channel is connected to a source (in the upper left corner of the ETS-7000), power is available at each post in the channel. There should be wires connecting all of the red channels together and all of the blue/black channels together. If these wires are not already on your board, you can add them as you go. Again, don't get red and black reversed.
Chips: We'll use three kinds of chips today: 7404 (NOT gates), 7408 (AND gates), and 7432 (OR gates). Data sheets describing the operation of these chips are attached. As the semester goes on, you'll be using lots of other chips. Data books on the shelf in the lab describe their operation. When possible, data sheets will also be posted on the class web pages, under the Documents section. Some important notes on the chips and their use:
\Orientation: Each chip has a mark, either a small hole or a small cutout, to indicate the top end. The top pin on the left is pin 1. The pin numbers go down the left side and up the right side. Most of our chips have 14 pins, but some will have more.
Power and ground pins: Two pins on each chip must be connected to power channels. The pin marked Vcc on the data sheet should be connected to the red channel. The pin marked GND should be connected to the black/blue channel. Don't get them backwards, and don't install the chip upside down.
Installing chips: Chips must be laid on the board with the right end up and with the pins straddling the groove. Each row of five pins is electrically connected within the board. By placing the chip so it straddles the groove, you are ensuring that there are separate sets of holes for each pin of the chip. Be sure to press each chip in gently and evenly, without letting one end or the other twist up or down. This will help you avoid breaking pins.
Removing chips: Remove chips carefully and evenly, by using a chip extractor and pulling straight up. Do not try to remove the chips with your fingers, as you'll probably bend or break the pins in the process. Return each chip to the place you got it.
Wires: There are bins of wires available for your use, as well as kits that contain pre-cut, pre-bent wires that are color-coded by length. When you make a connection on the board, try to do it neatly and try to avoid using wires that are too long for the purpose. Your work will be far easier to test and correct if the wires are short, well organized, and labeled. When you need a wire of a certain length, you can either look for one or you can make one from a longer wire, using wire strippers to cut and remove insulation. You may even want to devise for yourself a wire-color scheme, such that each color implies something about the purpose to which the wire is being put.
Logic probes: Logic probes are great debugging tools. If you connect them to power you can insert the probe tip into holes to determine if there is a positive voltage (a red light and a high-pitched sound), a ground connection (a green light and a low-pitched sound), or an open connection (no light or sound). Probes don't always work in locations that are connected to LEDs. As an alternative, you can always use a wire connected to one of the ETS-7000's LEDs as an ad-hoc logic probe.
We may not use all of these parts in the first lab, but you will need a number of them, and eventually you will use all of them and more.
It is unwise to construct a complete solution without testing its parts along the way. With a large circuit, you want first to construct the smaller circuits that will later compose the large one. With each smaller circuit, you should debug it thoroughly before connecting it to other smaller circuits. That way, you can have some confidence that certain portions of your larger circuit work correctly, reducing the number of possible portions of your circuit that may be designed or constructed in error.
Today, we'll move one part at a time through constructing the half-adder:
Be sure to leave yourself a little time to complete the lab! Every week, there are some things you'll have to do whether or not you've completed the assignment.